1. Field of the Invention
This invention relates to electronic circuits and, more particularly, to a transversal filter having programmable tap weights.
2. Description of the Prior Art
FIG. 1 shows a block diagram of a transversal filter. An analog input signal X(t) to be filtered is applied to terminal 11-0. This input signal X(t) is multiplied by tap weight a.sub.0, and the result of this multiplication is input to summation means 21. The input signal X(t) is also applied to delay means T.sub.1, thus providing at time t an output signal X(t-1) on node 11-1. This analog signal X(t-1) is multiplied by tap weight multiplier a.sub.1, and the result is applied to summing means 21. The analog signal X(t-1) available on node 11-1 is also applied to delay means T.sub.2, thus providing at time t an analog signal X(t-2) on node 11-2. This analog signal X(t-1) is multiplied by tap weight multiplier a.sub.2 and the result is applied to summing means 21. In a similar manner, a plurality of N delay means T.sub.1 through T.sub.N (where N is selected positive integer) are connected as shown in FIG. 1, with the output signal from each delay means being defined as: EQU V.sub.j (t)=X(t-j) (1)
where
V.sub.j (t)= the output signal from the jth delay means at time t, where 0.ltoreq.j.ltoreq.N; and PA1 X(t-j)= the input signal applied to input terminal 11-0 at time (t-j).
The output signals from each of the N delay means T.sub.1 through T.sub.N are multiplied by tap weights a.sub.1 through a.sub.N, respectively, and the non-delayed input signal X(t) is multiplied by tap weight a.sub.0. In this manner, the output signal y(t) on output terminal 12 of summing means 21 is equal to: ##EQU1##
One such prior art transversal filter is described by Puckette et al. in "Bucket-Brigade Transveral Filters", IEEE Transactions on Communication, Volume COM-22, No. 7, July 1974, pages 926-934. Puckette et al. describe the use of a bucket brigade delay line in a transversal filter. The Puckette approach taps the delay line with source followers, the desired weighting being established by the appropriate choice of capacitance values within each source follower.
Transversal filters have also been implemented utilizing charge-coupled devices (CCDs). Such a CCD transversal filter is described by Brodersen et al. in "A 500-Stage CCD Transversal Filter for Spectral Analysis", IEEE Journal of Solid-State Circuits, Volume SC-11, No. 1, Feb. 1976, pages 75-83. Broderson et al. show that the tap weights are established by the use of appropriate photomasks for forming the electrical interconnects on the device surface. Another prior art CCD transversal filter is described by Baertsch et al. "The Design and Operation of Practical Charge-Transfer Transversal Filters", IEEE Journal of Solid-State Circuits, Volume SC-11, No. 1, Feb. 1976, pages 65-73.
The above-mentioned prior art transversal filters are not programmable in that the tap weight multipliers, once established, may not be altered to provide a different transversal filter characteristic. Attempts have been made to provide a programmable transversal filter. One such attempt is described by White et al. in "CCD and MNOS Devices for Programmable Analog Signal Processing and Digital Non-Volatile Memory", IEEE IEDM, Washington, D.C., 1973, pages 130-133. White et al. utilize a programmable MNOS conductance which is programmed with a train of pulses to adjust the threshold voltage.
Another programmable transversal filter is described by Haque & Copeland in "An Electrically Programmable Transversal Filter", International Electron Devices Meeting, Dec. 1976, pages 27-30. This prior art programmable transversal filter operates by cycling the tap weight coefficients in a digital shift register. However, such a technique results in the generation of fixed pattern noise due to inherent irregularities in the capacitance of the capacitors used to fix the tap weights stored in the digital shift register.